This invention relates to improvements in spark ignition systems of internal combustion engines having the capabilities of improving engine operation by permitting combustion of extremely lean fuel mixtures, more accurately setting and controlling spark timing, and further enabling ignition system components to be designed more efficiently.
In order to initiate combustion of an air/fuel mixture within an internal combustion engine chamber, a spark ignition system is used which generates a high energy arc at the appropriate time in the engine operating cycle. The onset of the arc across a spark plug gap is timed to occur at a predetermined number of degrees of engine crankshaft rotation before the piston reaches top dead center (TDC). If spark timing is established properly, the flame front emanating from the spark plug will cause a pressure peak to develop within the combustion chamber which occurs just after top dead center of the piston during its power stroke. If the spark is initiated too late in the operating cycle (retarded timing) the developed pressure within the combustion chamber will not be efficiently converted to work output. On the other hand, if the spark is initiated too early in the cycle (advanced timing), extemely high and potentially damaging pressure and temperature rises can occur in the combustion chamber which are also not efficiently converted into useful work.
Excessively advanced spark timing can lead to several different types of combustion chamber phenomena. Auto-ignition of the end gases is a condition where the end gases (the unburnt fuel-air mixture that is being ignited by the movement of the flame front) explodes spontaneously when the engine combustion temperatures and pressures become too high. When auto-ignition occurs in the cylinder of the spark engine, pressure therein rises and falls alternately due to the sudden release of chemical energy and temperature rapidly increases. If the rate of energy release is sufficiently high, vibrations within the exploding gas will force the cylinder walls to vibrate, resulting in a characteristic sound referred to as "pinging", or other audible sounds. The rapid fluctuations in pressure and temperature of gases within the combustion chamber caused by auto-ignition occur well after top dead center.
A slight degree of auto-ignition is believed by many engine designers to be desirable because it generates turbulence which hastens the combustion process at a time when the speed of the flame emanating from the spark plug is decreasing. Slight auto-ignition can reduce hydrocarbons left unburnt by the spark-triggered ignition process and simultaneously utilizing the energy released when they are burnt, resulting in lower hydrocarbon emissions as well as improved fuel economy. For these reasons, engine designers often seek to calibrate ignition systems so that spark is advanced to about the threshold of auto-ignition. Care must be taken, however, to avoid excessive auto-ignition which leads to high combustion chamber temperatures which can eventually heat the spark plug electrodes to the point where they initiate the combustion process independently of the spark, thus leading to a phenomena known as pre-ignition. Pre-ignition is marked by extremely high cylinder temperatures and pressures near TDC and can cause significant engine damage, including perforation of the piston. Pre-ignition is frequently referred to as "knock" due to the characteristic audible sound which it generates. Generally, it can be stated that auto-ignition leads to pre-ignition, and subsequently, pre-ignition leads to furthe auto-ignition.
A number of factors influence the timing threshold of generating auto-ignition, including inlet air temperature, engine speed and load, air/fuel ratio, fuel characteristics, and a host of other variables. Spark timing further directly affects engine fuel efficiency and noxious emissions output. Due to the significance of accurately controlling spark timing, numerous engine control systems in present use have microprocessor based closed-loop spark timing control systems which simultaneously measure a number of parameters such as exhaust composition, coolant temperature, and the occurrence of spark knock. These systems proces these data to set timing to near a predicted auto-ignition threshold. The present spark knock detectors used with spark controllers are typically a piezoelectric transducer which senses the intense vibrations caused by spark knock. These knock detectors, however, are not sensitive enough t detect incipient engine auto-ignition which may produce a barely detectable engine vibration and therefore the threshold of auto-ignition is not sensed by such transducers. Accordingly, there is a need to provide a spark ignition control system which enables the detection of incipient auto-igniton, thus enabling more precision in setting spark timing in a closed-loop system.
Designers of spark ignition internal combustion engines for motor vehicles are constantly striving to enable the engines to burn leaner air/fuel mixtures (i.e., lower fuel concentration). An air/fuel mixture of approximately 15 to 1 (respectively) is referred to as a stoichiometric mixture and provides just enough oxyge to completely burn the fuel charge. Adding excess air to the combustion chamber, however, has been found to reduce noxious engine emissions such as oxides of nitrogen and hydrocarbons, etc. There are limits, however, to the extent to which the mixture can be leaned before the spark will not produce an exothermic reaction within the combustion chamber. The presen lean limit for most present motor vehicle engines is approximately 20 to 1 air/fuel ratio. Engine designers are striving to increase the lean burn limit of engines, which is theoretically believed to be extendable to about 27 to 1 air/fuel ratio. There is a need therefore to extend the lean limit of spark ignition internal combustion engines.
In newer generation ignition systems, a transformer generally known as an ignition coil is mounted directly on each of the ignition spark plugs and is often referred to as a coil-on-plug (COP) ignition arrangement. The size and mass of such devices placed on the spark plugs is greatly affected by thermal requirements. Windings which operate with high duty cycles (i.e., periods of winding energization cmpared with dwell periods) must be large and massive enough to prevent excessive heating of the winding. Conversely, low duty cycle operation enables the winding to be made more compact and lighter in weight. Reductions in size of COP windings is further desirable to reduce engine packaging constraints. There is accordingly a need to provide an ignition system having a device mounted on the spark plug which is of minimum size and weight.